The present invention relates to a method of fabricating an optically nonlinear thin film waveguide using a glass substrate, and to an optically nonlinear thin film waveguide. More particularly, the present invention relates to controlling the shape of a waveguide having optical nonlinearity.
Optical functional elements utilizing second-order optical nonlinearity are known. While such elements are usually formed of a crystalline material, optical fibers are formed of a glass material. Considering cost and compatibility with an optical fiber, a need exists for fabricating the optical functional element of a glass material. In addition, as a planar element is suitable for achieving various optical control (signal control) functions, an optical functional element formed of a glass substrate is desired.
A method of fabricating a planar optical waveguide of a glass material is disclosed in Japanese Patent Laid-Open Publication No. Hei 8-146475. According to the method disclosed in that document, a glass film dispersed with fine particles is deposited on a glass substrate, and a resist mask is formed of photoresist over a portion to serve as a core. Next, the portion of the particle dispersed glass film that is not covered by the resist mask is removed by reactive ion etching to form an optical waveguide (core) portion. After removing the resist mask, a glass film is deposited to surround the core, and this portion serves as cladding. High power laser light is irradiated to a part of the core portion of the thus formed optical waveguide, thereby imparting high optical nonlinearity to the irradiated part.
Although the above-described method of fabricating a planar optical waveguide requires an etching step for leaving the portion of the resist film corresponding to the core portion, etching the film to leave only the core is difficult because this portion is thin. Also, optical nonlinearity obtained by this method is third-order nonlinearity, not second-order nonlinearity. Consequently, only a small nonlinearity is obtained, and therefore it is difficult to implement an element which sufficiently operates as an optical element.
The applicant proposed a method of manufacturing a planar optical waveguide by UV-excited poling in Japanese Patent Application No. Hei 8-244965. According to this method, a pair of electrodes are formed on the surface of a glass substrate. Using the electrodes as a mask, a gap portion between the electrodes on the surface of the glass substrate is doped with germanium (Ge), so that the portion serves as a core. By applying a high voltage across the gap between the electrodes while irradiating ultraviolet rays, the core is subjected to ultraviolet poling and given second-order optical nonlinearity. The optical nonlinearity induced by UV poling is substantially as high as that of a crystalline material, such as LiNbO3, and therefore the planar waveguide thus obtained can be used for forming a wide variety of functional optical waveguides.
It should be noted that single optical mode propagation and operation are essential to functional optical waveguides, such as optical switches and optical modulators used for optical communication, optical measurement, optical information processing, or the like. When a plurality of optical modes are present, propagation constants of the respective modes (refractive indices for the respective modes) are different, and therefore operation voltages for switching or the like utilizing optical interference effect will be different. Thus, in order for an optical waveguide to achieve operation such as switching, the waveguide is required to have a shape that allows single mode optical propagation.
The shape of an optical waveguide is determined by a combination of its refractive index and three dimensional size. According to the applicant""s method outlined above, the thickness of an optical waveguide (depth from the surface of the substrate) formed by UV-excited poling is controlled using optical absorption of the substrate to change the intensity of ultraviolet radiation.
In order to achieve a single mode, the optical waveguide must have the smallest possible size, and the light intensity of ultraviolet radiation must be decreased for this purpose. However, the shape of the waveguide and the induced optical nonlinearity cannot be controlled independently from each other because the induced optical nonlinearity is also dependent on the light intensity of ultraviolet radiation. Further, higher optical nonlinearity is obtained as the intensity of ultraviolet radiation increases. Consequently, the size of the waveguide is increased for the sake of imparting a high optical nonlinearity, making it impossible to achieve single mode propagation.
The present invention aims to provide an optically nonlinear waveguide and a method of fabricating the optically nonlinear thin film waveguide wherein the optically nonlinear waveguide formed of a glass material has a sufficiently high second-order optical nonlinearity, and allows a proper three-dimensional shape to be obtained.
The fabricating method according to the present invention comprises the steps of forming a thin SiO2 film containing Ge on a glass substrate, forming thin metal electrode films on the thin SiO2 film with a gap between the electrode films having a shape corresponding to a waveguide pattern, and irradiating the Ge-containing thin SiO2 film with ultraviolet radiation through the gap while applying a voltage across the gap between the thin metal electrode films.
Thus, the thin SiO2 film containing Ge is formed on the glass substrate, and, therefore, it is limited to the Ge-containing thin SiO2 film where second-order optical nonlinearity is induced by UV-excited poling. As a result, the width can be defined by the shape of the electrodes, and the depth can be controlled by the thickness of the Ge-containing thin SiO2 film, so that the shape of the optically nonlinear waveguide can be controlled in three dimensions. A single mode propagation can be achieved in the optically nonlinear waveguide, to thereby ensure operation, such as switching, in the optically nonlinear waveguide. While the glass substrate is preferably formed of SiO2 glass, other materials, such as sodium glass, can also be employed.
The method of fabricating an optically nonlinear thin film waveguide according to the present invention may further comprise the steps of providing a thin transparent insulating film on the thin metal electrode films to cover at least said gap portion, and irradiating the Ge-containing thin SiO2 film with ultraviolet radiation through the gap between thin metal electrode films while applying a voltage across said gap. Such provision of the insulating film can prevent electric discharge which would otherwise be caused by dielectric breakdown during application of a voltage across the gap between the metal electrodes for UV-excited poling. The thin insulating film must be formed of a material having a high breakdown voltage and transmitting ultraviolet radiation, preferably SiO2.
Preferably, the optically nonlinear thin film waveguide is formed in a vacuum chamber. Because dielectric breakdown does not occur in a vacuum as it does in air, a sufficiently high voltage can be applied across the electrodes for UV-poling.
An optically nonlinear thin film waveguide according to the present invention includes a thin SiO2 film containing Ge and formed on a glass substrate, and thin metal electrode films formed on the Ge-containing thin SiO2 film with a gap between the electrode films having a shape corresponding to a waveguide pattern, wherein the portion of the Ge-containing thin silica film corresponding to the gap between the thin metal electrode films exhibits second-order optical nonlinearity.
The optically nonlinear thin film waveguide according to the present invention may further include a thin transparent insulating film formed on the thin metal electrode films to cover the gap.